The present invention relates to a device useful for a direct spectrum spread receiver in which a transmission signal, obtained by modulating a carrier of, e.g., the UHF band including an information signal in accordance with a spreading code to spread its spectrum to a wide band, is subjected to reverse-spreading modulation to restore the original information signal. The reverse-spreading modulation is performed using a code identical to the spreading code such that the modulation is in synchronization with the spreading code in the received transmission signal. The invention particularly relates to a device of this type which can automatically be calibrated so as to compensate for characteristic changes (drifts) in the receiver circuit due to a temperature change or time lapse.
FIG. 2 shows a receiver circuit of the conventional spectrum spread system. A transmission signal received by an antenna 3 is applied to an RF amplifier 1 and then applied to a correlator 2 which is constituted by amplifiers 21, 22 and 23, multipliers 31, 32 and 33, correlating filters 41, 42 and 43, detectors 51, 52 and 53, a differential amplifier 60, and a comparator 61. The circuit of FIG. 2 further includes a loop filter 7, a voltage-controlled oscillator (VCO) 8, a pseudo noise generator 9, and a shift register 10. The shift register 10 receives a pseudo noise signal (PN signal) from the generator 9 and produces delayed PN signals delayed by 0, 1/2or 1 chip to be applied to the multipliers 32, 33 and 31, respectively.
When the gain of an amplifier or a DC potential is changed due to a drift, i.e., a characteristic change of circuit elements due to a change in ambient temperature or time lapse, a voltage at the output point C of the detector 53 is changed, causing such erroneous detection that an appropriate correlation detecting signal S.sub.D cannot be obtained, or that a detecting signal S.sub.D is produced when it is not to be produced. Further, even if the correlation detection is performed correctly, there may be the case that an offset voltage or a gain difference appears in the output signals of the detectors 51 and 52, i.e., input signals to the differential amplifier 60, so that a lock point of a delay lock loop is shifted.
FIGS. 3A through 3C are graphs showing the relationship between input signals and an output signal of the differential amplifier 60. FIG. 3A shows the relationship between the output of the differential amplifier 60 and voltages at the correlation detection points a and b in the ideal case in which no drift occurs. On the other hand, in FIG. 3B a gain reduction of an amplifier is reflected in the correlation output at b, and in FIG. 3C a DC level change has occurred in the correlation output at b. As a result, there occurs a phase shift D in the output of the differential amplifier 60, and a tracking point F is shifted to the phase-delaying side in both cases of FIGS. 3B and 3C.